At a base station of a digital cellular radio system, typically all timing and clock signals are derived from a master clock signal generated locally at the base station. The stability requirement for the master clock signal is 0.05 ppm according to the recommendations of the GSM system, for instance. One approach to meet this stability requirement is to use as master oscillator of the base station a very accurate crystal oscillator, positioned in a space at a stable environmental temperature, a so-called oven. This kind of crystal oscillator is generally called oven stabilized crystal oscillator. A sufficiently accurate oven stabilized crystal oscillator is, however, a very expensive solution, in which long-term instability is inevitable. Consequently, the oven stabilized crystal oscillator must be regularly calibrated to the assigned frequency during operation.